Comparable dynamic cerebral autoregulation and neurovascular coupling of the posterior cerebral artery between healthy men and women

Abstract Aims Most studies focus on dynamic cerebral autoregulation (dCA) in the middle cerebral artery (MCA), and few studies investigated neurovascular coupling (NVC) and dCA in the posterior cerebral artery (PCA). We investigated NVC and dCA of the PCA in healthy volunteers to identify sex differences. Methods Thirty men and 30 age‐matched women completed dCA and NCV assessments. The cerebral blood flow velocity (CBFV) and mean arterial pressure were evaluated using transcranial Doppler ultrasound and a servo‐controlled plethysmograph, respectively. The dCA parameters were analyzed using transfer function analysis. The NCV was evaluated by eyes‐open and eyes‐closed (24 s each) periodically based on voice prompts. The eyes‐open visual stimulation comprised silent reading of Beijing‐related tourist information. Results The PCA gain was lower than that of the MCA in all frequency ranges (all p < 0.05). Phase was consistent across the cerebrovascular territories. The cerebrovascular conductance index (CVCi) and mean CBFV (MV) of the PCA were significantly higher during the eyes‐open than eyes‐closed period (CVCi: 0.50 ± 0.12 vs. 0.38 ± 0.10; MV: 42.89 ± 8.49 vs. 32.98 ± 7.25, both p < 0.001). The PCA dCA and NVC were similar between the sexes. Conclusion We assessed two major mechanisms that maintain cerebral hemodynamic stability in healthy men and women. The visual stimulation‐evoked CBFV of the PCA was significantly increased compared to that during rest, confirming the activation of NVC. Men and women have similar functions in PCA dCA and NCV.


| INTRODUC TI ON
Cerebral autoregulation (CA) is defined as the ability to adjust the caliber of small cerebral vessels to change vascular resistance and maintain a relatively constant cerebral blood flow (CBF) when the average arterial pressure fluctuates. 1 It is also an important method for preventing over-or underperfusion of brain tissue.CA can be divided into two main types: static CA (sCA) and dynamic cerebral autoregulation (dCA).The former refers to changes in CBF before and after a slow change in blood pressure (BP), reflecting the overall regulatory ability of the brain and its upper and lower regulatory limits.Due to its inability to reflect the impact of BP fluctuations on CA capacity over a short time and the frequent need for pharmacological interventions to change BP, sCA has limited clinical application. 1,2rrently, more researchers are focused on dCA, which describes the immediate response of CBF to rapid BP changes.In 1982, with the advent of transcranial Doppler (TCD) ultrasound, the sustainable measurement of cerebral blood flow velocity (CBFV) brought about a revolution in the field of CA, achieving real-time hemodynamics analysis. 3The TCD has become the most widely used technology for evaluating the dCA function. 4[7] Therefore, reliable assessment of dCA may facilitate the understanding of clinical severity and guide personalized patient care.However, most previous studies on regional dCA have focused on the middle cerebral artery (MCA), ignoring potential regional heterogeneity in blood vessels.Indeed, anatomical differences exist between the anterior and posterior circulation and in sympathetic nerve activity, cerebrovascular reactivity, and CBF. 8-10A recent review by Koep et al. suggested that future research should focus on distinguishing regional differences in dCA. 9 Notably, neurovascular coupling (NVC), one of the main mechanisms affecting cerebral hemodynamics, is a phenomenon in which the body changes regional CBF according to neural activity and metabolism. 11ile the effects of sex on CBFV, cerebrovascular reactivity to carbon dioxide, and dCA of the MCA have been extensively reported, 12,13 less attention has been paid to the effect of sex on dCA of the posterior cerebral artery (PCA) in the supine position using transfer function analysis (TFA).The TFA utilizes a fast Fourier decomposition of stationary input and output signals to decompose them into the sum of sines and cosines of multiple frequencies. 14Of note, the 2022 updated white paper of the cerebrovascular research network (CARNet) proposed that a dCA study design requires a balanced number of male and female participants, considering the impact of sex on the outcomes. 14nsequently, the present study explored dCA of the PCA using TFA methods and NVC and analyzed sex differences in dCA in healthy individuals to provide a basis for future research on dCA impairment.If there are sex differences in dCA, this may explain sex differences in, for example, patients with migraine.

| Assessment of the dCA
dCA assessments were performed by specialized vascular ultrasound physicians based on the recommendations of the International White Paper from CARNet. 14The examination of subjects was performed in a temperature-controlled environment of 22-24°C.Before measurements, participants had to (1) avoid alcohol, chocolate, and caffeine for at least 12 h; (2) refrain from moderate (or more intense) exercise for ≥6 h; and (3) avoid eating high-calorie food for at least 4 h.The participants rested for 15 min without crossing their legs before the dCA assessment, which was performed using a TCD (EMS-9D Pro; Delica Medical, Shenzhen, China).The CBFV of the MCA at a depth of 50-65 mm and of the PCA at 60-70 mm were measured using a 1.6-MHz probe in the bilateral temporal windows.Noninvasive continuous beat-to-beat BP (NIBP) was recorded using a finger photoplethysmograph sensor (Finometer®, Arnhem, The Netherlands).Before NIBP measurement, the BP was measured at the brachial artery using an Omron sphygmomanometer to correct baseline BP.The sampling frequency of the Doppler trace and NIBP signal was 125 Hz.In addition, a nasal cannula was connected to monitor end-tidal carbon dioxide (Et-CO 2 ).Continuous CBFV from the MCA and PCA, NIBP, HR, and Et-CO 2 were recorded in real time with the patient in the supine position for 10 min.

| Neurovascular coupling
The NVC of the PCA is mainly achieved through reliable and repetitive reading of visual stimulation, which is associated with per-

| Data analysis of dCA
In accordance with the recommendations of CARNet, we selected stable 5-min NIBP and TCD monitoring data from the recordings for dCA analysis with a Hanning window of 100 s in length and a 50% overlap.The TFA method was used to quantify the dCA parameters, including gain, phase, and coherence at very low frequency (VLF, 0.02-0.07Hz), low frequency (LF, 0.07-0.20 Hz), and high frequency (HF, 0.2-0.50Hz). 14 The average values of the dCA parameters in both hemispheres were used for further analyses.Phase reflects the temporal relationship between the BP (input signals) and CBFV (output signals), whereas the gain represents the amplitude at the same frequency.Coherence approaching 1.0 reflects a linear relationship between oscillations in BP and CBFV.Among them, a lower phase and higher gain reflect impaired dCA, and the phase is more reliable than other TFA parameters.If coherence is <0.5, the linearity condition may be violated in this frequency range. 16Thus, only dCA parameters with a coherence ≥0.5 were analyzed in the present study.
Regarding the frequency domain, the VLF and LF indices were of concern for dCA.

| Statistical analyses
All statistical analyses were performed using IBM SPSS software (version 27.0; Armonk, NY, USA).All continuous data were assessed for normal distribution using Kolmogorov-Smirnov tests.
Normally distributed continuous variables are expressed as means ± standard deviations and were compared using t-tests.In contrast, non-normally distributed data are presented as median with interquartile ranges and were compared using the Mann-Whitney U-test.
Categorical variables are expressed as percentages and were analyzed using the chi-square test.Group differences in MAP and MV of the PCA between resting and silent reading states were compared using a paired t-test.All statistical tests were two-sided, and statistical significance was set at p < 0.05.

| Demographical data
Sixty healthy volunteers were enrolled in the study, including 30 men and 30 age-matched women who completed the dCA and NVC assessments of the PCA.As shown in Table 1, the baseline MAP was higher in men than in women, and no significant differences in terms of age, body mass index, HR, and ET-CO 2 were found between the two groups.

| Comparison of the CBFV and dCA data between the MCA and PCA
The parameters of the CBFV and dCA data for the MCA and PCA are presented in Table 2.Among them, the MV of the MCA was higher than that of the PCA in the posterior circulation (60.68 ± 10.24 vs. 32.98 ± 7.25, p < 0.001).In terms of the dCA parameters, the PCA gain was lower than that of the MCA in all frequency domains (PCA vs. MCA, VLF: 0.62 ± 0.24 vs. 0.76 ± 0.21; LF: 0.68 ± 0.19 vs. 0.99 ± 0.22; HF: 0.71 ± 0.15 vs. 0.97 ± 0.22, all p < 0.001).The coherence of the PCA was also lower than that of the MCA in the LF and HF range (both p < 0.05).Phase was not statistically significantly different between groups.

| NVC measured by visual stimulation-evoked PCA MV and CVCi
Regarding NCV, the MAP was similar between the eyes-open and eyes-closed conditions.In contrast, PCA MV and CVCi were higher during visual stimulation (eyes-open) than when eyes were closed (both p < 0.001, Figure 1 and Table 3).The CBFV of the PCA was higher during silent reading than when the participants were at rest or had their eyes closed (Figure 2).

| A comparison of PCA dCA and NCV between men and women
As shown in

| DISCUSS ION
The present study investigated the two main mechanisms ensuring appropriate CBF in the PCA: dCA and NVC.We observed a significant increase in the visual stimulation evoked MV of the PCA compared to that during rest, confirming activation of NVC.And our study demonstrated that dCA (i.e., the gain, phase and coherence) and NVC response parameters (i.e., ∆CVCi and VEFR) for the PCA were similar between healthy men and women.Besides, we further explored whether regional differences in the dCA exist between the posterior and anterior circulation, and the results revealed that gain was lower in the PCA than in the MCA, indicative of a more effective hemodynamic autoregulation.In contrast, phase was similar in both the MCA and PCA.
As dCA is a highly complex mechanism, a great deal of research has been dedicated to methods for its noninvasive evaluation, such as multimodal pressure-flow analysis, correlation coefficient analysis, and the autoregulatory index (TFA). 17,18The TFA method uses a fast Fourier transform to decompose a stationary input signal (BP) and output signal (CBF/CBFV) and further quantifies dCA function.In 2016, a white paper from CARNet proposed standardization of the parameters and settings of the TFA method, which has become the most widely applied method for studying dCA. 14,17is study used TFA methods to evaluate dCA parameters of the anterior and posterior circulations.To date, most human studies on dCA have concentrated on sympathetic control of the anterior circulation.consistent with our findings.This may be related to baseline differences in CBFV. 22Current evidence suggests that differences in dCA be-  highlighted the need for more studies to gain a better understanding of the regional heterogeneity of dCA by assessing global and regional CBF in the future.
Numerous studies have reported sex differences in dCA of the MCA. 24,25Favre et al. 25 showed that there was no difference in dCA between men and women in the supine posture.However, there are contrasting data showing that dCA function in men is better than that in women in the supine posture. 26While the PCA is an integral part of the cerebrovasculature, it remains poorly understood whether sex has an effect on dCA in the posterior circulation.Currently, similar to the MCA, the literature regarding the effect of sex on dCA in the PCA shows inconsistent results. 21,27,28vilala et al. 27 observed that girls had a higher autoregulatory index (ARI) in the basilar artery than healthy boys (a higher ARI indicates greater dCA).Conversely, Tontisirin et al. similarly applied ARI to quantify autoregulatory functions and found no sex differences in dCA of the MCA or basilar arteries in prepubertal children. 28Similar to our results, a recent study, which also used the TFA method to assess dCA in healthy subjects, demonstrated that men and women have similar dCA in the PCA in terms of spontaneous oscillations in BP. 21They further assessed dCA driven by squat-stand maneuvers and reached the same conclusion.

TA B L E 4
Comparison of dCA parameters of PCA between different genders.
Thus, further investigations are required given the differences in hormone levels between the sexes and the different methods of evaluating dCA.
NVC, the coupling between local neuronal activity and changes in CBF, causes an increase in regional CBF to meet metabolic demands, mainly through a series of regulatory activities by cells such as astrocytes. 15 which may be selectively activated. 29Additionally, CBFV can be increased by releasing relevant neurotransmitters and transmitting vascular activity signals. 29,30Previous studies have shown that NVC is impaired in clinical populations, such as those with highlevel spinal cord injury and stroke. 31,32This study focused on the normative values of NVC in healthy humans, which can provide a reliable basis for future research to determine the impairment of NVC.Moreover, we further explored whether sex differences can confound NVC assessment, as this is not well studied.The existing literature, consistent with our application of a similar methodology and TCD assessment of NVC, also revealed that men and women have similar response magnitudes for NVC. 33,34Interestingly, a recent study by Leacy et al. showed that the time from visual stimulation to the peak NVC response is shorter in women and that differences in NVC response integration speed may be due to sexrelated differences in brain network connectivity. 33,35This should be studied further in the future.
Finally, this study had certain limitations.First, the current study is based on the assumptions that the intracranial artery diameter remains constant and that TCD mainly measures CBFV rather than CBF, and then further quantifies the dCA and NVC.Previous studies found that the diameter of the artery did not change significantly in response to physiological stimulation.Therefore, CBFV can be used as a reliable surrogate for CBF. 36In addition, given the single-center study design with a relatively small sample size, future studies with larger sample sizes are needed to improve our understanding of dCA and NVC.

| CON CLUS ION
This study explored regional heterogeneity of the anterior and posterior circulation dCA.We showed that the main vessels within the

ACK N OWLED G M ENTS
We thank the staff associated with the study and all the patients and their families for their cooperation.The graphical abstract in the manuscript was drawn using Figdraw.

FU N D I N G I N FO R M ATI O N
This study was funded by the National Natural Science Foundation cultivation project of Xuanwu Hospital (No. QNPY2022007).

CO N FLI C T O F I NTER E S T S TATEM ENT
The authors report that there are no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.

PATI ENT CO N S ENT S TATEM ENT
All procedures were verbally explained in detail to the volunteers, and written informed consent was obtained.

Ran
fusion in the occipital visual cortex.Participants were assessed during a 2-min baseline period with eyes-closed and six repeated cycles of 24 s with eyes-open (silent reading-visual stimulation) followed by 24 s with eyes-closed.The visual stimulus consisted of reading material in Chinese related to general tourist information about Beijing.The mean CBFV (MV) of the PCA and mean arterial pressure (MAP) at rest and during each cycle during silent reading were measured, and cerebrovascular conductance index (CVCi) = PCA MV/MAP, 15 and visually evoked flow response (VEFR) = (MV-silent reading -MV-rest)/MV-rest × 100% were calculated. 11 brain regions are physiologically plausible because these heterogeneities may reflect different sympathetic and parasympathetic innervation in the anterior and posterior circulation.At the same time, different functional and anatomical features also exist between these two territories. 9,23In their review, Koep et al. 9 also F I G U R E 1 Comparison of NVC parameters between eyes-closed (resting) and eyes-open (silent reading).(A) The mean cerebral blood flow velocity (MV) and (C) the cerebrovascular conductance index (CVCi) of the posterior cerebral artery (PCA) were both significantly higher during the eyes-open than the eyes-closed period.(B) The mean arterial pressure (MAP) was similar between the eyes-open and eyesclosed conditions.*p < 0.05.
cerebrovascular system (MCA and PCA) have similar phase, which quantifies the time course of hemodynamic regulation.However, the amplitude of dCA (i.e., gain) was lower in the PCA, indicative of a tighter hemodynamic autoregulation.Furthermore, we found that sex did not affect dCA and NVC responses in the PCA in healthy participants.Our study provides reliable normative values for dCA and NVC for the PCA in healthy men and women, forming a basis for further research.AUTH O R CO NTR I B UTI O N SYX and HC conceived and designed the study.HC, SC, LC, RL, and XP contributed to the dCA and NVC data collection and methodology.HC and FZ performed the statistical analyses.HC wrote the manuscript.Prof. YX edited and critically revised the manuscript.Each authors read and approved the final version of the manuscript.
Healthy volunteers were consecutively recruited from the Capital Medical University Xuanwu Hospital between June 2022 and November 2022.This study was approved by the Ethics Committee of the Capital Medical University Xuanwu Hospital (approval number: [2021]109).Individuals were eligible if they were ≥18 years old.
Demographic characteristics of participants.Comparison of dCA parameters between the middle cerebral artery and the posterior cerebral artery.
Note: Data are presented as mean ± SD.Abbreviations: BMI, body mass index; Et-CO 2 , end-tidal carbon dioxide; MAP, mean arterial blood pressure.TA B L E 2Abbreviations: HF, high frequency; LF, low frequency; MCA, middle cerebral artery; MV, Mean cerebral blood flow velocity; PCA, posterior cerebral artery; VLF, very low frequency.
Comparison of NVC parameters between resting and silent reading.
TA B L E 3 21 11D, with its high temporal resolution and noninvasive nature, can not only record CBFV but can also monitor the changes and responses of the vascular system induced by internal or external stimuli.It has become a commonly used and useful tool for evaluating NVC.11According to published NVC guidelines,